The present invention is related generally to the field of computer graphics, and more particularly, to a memory system and method for use in a computer graphics processing system.
Graphics data is typically stored in memory in a one-dimensional (1D), or linear, fashion, although data is often conceptually depicted in two-dimensions (2D). For example, a texture map is conceptually represented as having two-dimensions (u, v), but in actuality, the texture data is stored in memory such that the memory addresses for the texture map data are sequential. In accessing the texture data, the requested memory addresses are incremented to access the texture data in a row-like fashion. When the address for the texture data for the last texel in a row is accessed, the memory address is again incremented to roll over and access the texture data for the first texel in the next row.
Typically, data that is requested for processing is retrieved from memory in blocks and temporarily stored for processing in memory pages. In the case of a texture map, each memory page typically includes the texture data for one row of the texture map. When additional data is requested for processing, the memory page of data is first checked for the requested data before performing a memory access operation. If the requested data is already present in the memory page, then overall processing time is reduced. However, if the requested data is not present in the memory page, a xe2x80x9cpage missxe2x80x9d occurs, and another block of data including the requested data is retrieved from memory to replace the former memory page of data. Page misses add to the overall processing time, and consequently, it is desirable to avoid frequent page misses.
A problem with storing 2D data, such as texture data, in a 1D arrangement is that when bilinear interpolation is performed to determine the color value of a destination pixel in a graphics image on which the texture map is applied, the texture data for four texels from two different rows are required. In bilinear filtering, the color values of four texels closest to the respective location of the pixel are weighted and a resulting color value for the pixel is interpolated from the color value of the four texels. The four texels closest to the destination pixel are naturally arranged in a two-by-two square, and are thus occupy two different rows. As previously discussed, the texture data for two different rows are located on two different memory pages. Consequently, when the texture data for the four closest texels to the destination pixel are retrieved, an average of two page misses will occur: once when the data for the two texels from the first row are retrieved, and once again when the data for the two texels from the second row are retrieved. The page misses slow down the processing of the texture data because the data from one page is purged when the texture data of another page is loaded.
An approach to reducing the occurrences of page misses where bilinear interpolation is applied is to implement a xe2x80x9ctwo-dimensionalxe2x80x9d memory array such that the memory is conceptually segmented into several small 2D arrays. In this fashion, although the width of a texture map is divided into several 2D segments, the texture data for texels of several adjacent rows may be stored on a common memory page. Thus, the number of page misses occurring during texture application is reduced. Another approach to reducing page misses is disclosed in U.S. patent application Ser. No. 09/515,246, entitled METHOD AND SYSTEM FOR ADDRESSING GRAPHICS DATA FOR EFFICIENT DATA ACCESS to Peterson, filed on Feb. 29, 2000. The approach described therein maps texture data of a texture map into virtual two-dimensional memory arrays but is implemented in a one-dimensional memory space. An offset value is determined from the (u, v) texel coordinates for the requested texels, and from the offset values, a virtual 2D memory address is calculated and used to effectively store in a 1D memory space the texture data in virtual 2D arrays. As a result, where texels of two different rows of the texture map are required for bilinear filtering, the arrangement of the texels in the virtual 2D memory arrays facilitates texel data processing and minimizes the occurrences of page misses. Although the methods described solve much of the problem with page misses, data is mapped according to a 2D arrangement regardless of the type of data being stored. In cases where it is more efficient to store the data in a 1D fashion, the use of the virtual 2D addressing actually creates inefficient data storage.
The present invention is directed to a memory system and method for accessing graphics data in a computer graphics processing system. The memory system includes a plurality of memory arrays, each of which has address terminals to receive requested memory addresses corresponding to memory locations within the respective array. A format register having a programmable format flag is further included in the memory system. The status of the format flag indicates which of a plurality of memory address allocation formats the memory addresses for each of the memory arrays are allocated. An address decoder is coupled to the format register to obtain the status of the format flag in order to determine the address allocation method for an array being accessed. The address decoder is further coupled to receive a requested address for a memory location in one of the memory arrays and then provide a requested memory address to the address terminals of the memory arrays. The requested address is translated to the requested memory address by the address decoder according to the memory address allocation format indicated by the format flag status for the memory array.